1. Field of the Invention
The present invention relates to an electromagnetic flowmeter for measuring a flow rate of fluid flowing in a flow tube in a non-filling state.
2. Description of the Related Art
An electromagnetic flowmeter applies a magnetic field which is perpendicular to the axis of a tube. As an electrically conductive fluid flows through the tube, an electromotive force is generated. The electromotive force is detected by two electrodes arranged in the tube. The flow rate of the fluid is calculated from the electromotive force thus detected.
Generally, a position of the electrodes is selected suitably so that the electromotive force thus detected is proportional to the flow rate. More specifically, the electrodes are arranged symmetrically with respect to a line of the axis of the tube. Since the electrodes are so arranged, the influence with respect to the inter-electrode electromotive force (i.e., a combination of electromotive forces generated at various parts of the fluid) is minimized when the tube is filled up with the fluid.
The electromotive force generated when the electrodes are appropriately positioned is given as follows: ##EQU1## where
W: weight function of the influence on the inter-electrode electromotive force,
x, y: coordinates of the surface which is perpendicular to the axis of the tube,
B: magnetic flux density,
v: flow velocity.
The equation (1) shows the weight function W has a good symmetry. This means that magnetic field is distributed uniformly in an ideal laminar flow of the fluid, making no errors in measuring the flow rate.
In practice, the flowing fluid does not always fill up the tube. Even if the fluid flows in a non-filling state, the conventional flowmeter in which the electrodes are immersed in the flowing fluid can measure the flow rate of the fluid. However, the weight function W greatly differs in shape from the weight function detected when the fluid flows in the filling state. The weight function W has but a poor symmetry. The poor symmetry means changes in distribution of flow rate in the tube, which result in errors in measuring the flow rate.
Moreover, the electrodes are arranged at the position corresponding to the height of the center of the flow tube. Due to this, when the level of fluid flowing in the flow tube is 50% or less, the electrodes are exposed from fluid. As a result, electromotive force cannot be measured.
To solve the above problems, the following electromagnetic flowmeter is proposed.
Specifically, in the proposed electromagnetic flowmeter, a magnetic filed is generated to be perpendicular to a ring earth electrode and a point electrode, thereby measuring electromotive force of an electrode tube. In this case, the ring earth electrode contacts fluid at an end portion of the tube, and the point electrode is attached to a lower portion of the tube.
However, in such an electromagnetic flowmeter, electromotive force strongly depended on the fluid level. This was a cause that the flow rate error occurred in measuring the flow rate. Also, measures against dependence on the fluid level in electromotive force were not fully taken.